1. Field of the Invention
This invention relates to an exhaust gas purifying catalyst for simultaneous removal of carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxides (NOX), which are harmful component contained in the exhaust gas from an internal combustion engine, and a method for the purification of exhaust gases by the use of the catalyst.
2. Description of Related Art
Various inventions have been proposed concerning the exhaust gas purifying catalyst for removing harmful components contained in the exhaust gas emitted from the internal combustion engine (JP-A-7-60117).
In recent years, various studies including the improvement of an engine itself are now under way with a view to complying with the global trend toward intensification of the regulations on the exhaust gas. One of the most leading methods is to expedite the ignition of a catalyst by approximating the position of the catalyst closely to the engine side thereby enabling the temperature of the catalyst bed immediately, after the start of the engine, to be elevated as quickly as possible. This method indeed excels in the ability to purify the exhaust gas immediately after the start of the engine but yet entails the disadvantage that it requires a catalyst having a higher heat resistant property because it is used in the proximity of the engine and exposed to a higher temperature than ever.
Technique of fuel cooling aimed at retaining the temperature of a material such as catalyst below a stated level has been adopted for the purpose of guaranteeing a part. In recent years, a demand for heat resistant property of the catalyst has been gaining because the fuel cooling tends to dwindle in response to the request for a lower fuel cost. In this circumstance, the conventional catalysts do not fully meet the demand for the heat resistant property.
We have pursued a diligent study for the purpose of fulfilling this object and taken notice of an oxygen storage material capable of adsorbing and desorbing oxygen, which is one of the most important components in three-way catalysts intended for use in gasoline vehicles. Various types of cerium oxide have been heretofore used as oxygen storage materials. To date, techniques such as addition of zirconium oxide, addition of the oxide of a rare earth element, or formation of a solid solution of cerium oxide in zirconium oxide have been disclosed with a view to enhancing the heat resistant quality of cerium oxide to store oxygen (Patent No. 2659796 in Japan). In these techniques, the ability to store oxygen per unit amount of cerium oxide reaches the maximum, when cerium oxide is doped into zirconium oxide, in a comparatively low range of 400xc2x0 C. to 500xc2x0 C.
Further, it has been ascertained that the abilities to store oxygen and heat resistant properties are enhanced more by doping cerium oxide into zirconium oxide in combination with some other rare earth oxides, particularly lanthanum oxide rather than by forming a solid solution of cerium oxide solely in zirconiumoxide. In the conventional oxygen storage materials, however, when cerium oxide or lanthanum oxide are doped in zirconium oxide, the components does not fully dope and thereby give birth to independent cerium oxide and the like, with the result that the produced solid solution assumes a heterogeneous crystal structure.
This invention has developed a catalyst having higher heat resistant property by using oxygen storage materials having a higher heat resistant property as described below and a method for purifying exhaust gases by the use of this catalyst. To be specific, the catalyst uses, as the oxygen storage material, a composite oxide with a homogenous tetragonal crystal structure of zirconium oxide, which is produced by doping cerium oxide and lanthanum oxide into zirconium oxide without forming independent cerium oxide or the like. Incidentally, it has been also ascertained that this composite oxide manifests higher oxygen storage ability at temperatures in the range of 400xc2x0 C. to 500xc2x0 C., even after treatments at temperatures exceeding 900xc2x0 C. in air, than the conventional oxygen storage materials.
The object of this invention, therefore, is accomplished by an exhaust gas purifying catalyst formed by coating a refractory three-dimensional structure with a catalytically active component containing at least one kind of noble metals, a refractive inorganic oxide, and a zirconium oxide containing cerium and lanthanum, characterized by the fact that the crystal structure of the zirconium oxide containing cerium and lanthanum is a tetragonal structure of zirconium oxide.
The object of this invention is also accomplished by a method for simultaneously purifying carbon monoxide, hydrocarbon, and nitrogen oxide contained in the exhaust gas emitted from an internal combustion engine, particularly a gasoline engine, by the use of the catalyst mentioned above.
According to this invention, carbon monoxide, hydrocarbon, and nitrogen oxide, which are contained in the exhaust gas emitted from the internal combustion engine such as the gasoline engine in an automobile, are simultaneously removed. Further, the catalyst of this invention excels in heat resistant property and durability because it comprises zirconium oxide containing cerium and lanthanum.
The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments.